This disclosure relates to an electrode assembly and method of forming same, as well as a discharge lamp such as a ceramic metal halide (CMH) lamp incorporating the electrode assembly.
If a leg temperature is high enough, seal corrosion is a leading failure mode for a discharge lamp such as a CMH lamp. In particular, material incompatibility is an issue associated with sealing a metal wire in a ceramic arc tube. That is, if the coefficients of thermal expansion of the respective materials are not sufficiently similar, then cracking ultimately results either in the seal glass or in the ceramic which leads to leakage of the dose and/or lamp failure. For example, it is known that alumina, a common ceramic used in CMH lamps, has a coefficient of thermal expansion that is a relatively close match with the coefficient of thermal expansion of niobium. This would tend to suggest using niobium as the sole material for the lead or electrode wire. However, niobium is incompatible with dose materials commonly used in CMH lamps. In fact, niobium will deteriorate in a matter of hours when exposed to the halide dose and ultimately leads to lamp failure. This suggests omitting niobium from use in the lead assembly. Tungsten and molybdenum on the other hand are more compatible with the dose materials. Tungsten and molybdenum suffer the problem of having coefficients of thermal expansion that are relatively incompatible with the alumina so that the mismatch in these materials leads to cracks in the ceramic material.
What has developed as a result of attempting to meet these competing concerns is a composite electrode assembly for a discharge lamp, and particularly for a CMH lamp, where the lead wire or electrode assembly is a composite of niobium at a first or outer end that is butt-welded to molybdenum as the intermediate or middle portion of the assembly and a tungsten electrode that is secured at the other end of the molybdenum. Moreover, the intermediate region of molybdenum is preferably comprised of two distinct portions, namely a molybdenum mandrel or shank that receives a molybdenum overwind, helix, or coil wrapped about it. In this manner, an opening through the leg is filled with an electrode assembly that is electrically conductive, thermally resistant, and resistant to the dose. The molybdenum mandrel with the molybdenum overwind has met these needs and the conventional thinking is that a tight winding was desired to fill the leg as completely as possible so that there is less of a region for the dose to condense or precipitate. That is, since a lamp leg is the equivalent of a cold spot, the leg has the drawback that in CMH lamps, for example, the dose condenses or precipitates in the leg. The first few milligrams of dose that are introduced into the discharge chamber ultimately end up in the leg, which becomes an expensive proposition. Thus, there has been a conventional desire to fill as much of the leg as possible with a thermally resistant, but electrically conductive, dose resistant material.
It is important to reduce the amount of seal voids in CMH lamps in order to abate the risk of decreased lamp life. Seal glass or frit seal is provided along at least a portion of the lead wire assembly to protect the niobium from the dose and also preferably extends inwardly along a portion of the molybdenum mandrel and helical overwind. It has been determined that voids are sometimes found in the structural arrangement and the seal voids are generally referred to as regions along the outer diameter of the molybdenum mandrel, and along an inner diameter region and between adjacent turns of the coil, that are devoid of frit seal (e.g., seal glass) or have pockets or openings, i.e., voids. The reason for formation of seal voids during the sealing process is not totally understood. However, a high variation of the amount of seal voids has been found within a single batch, as well as from one batch to another. Products whose lamp leg temperature is higher and/or have a higher amount of seal voids are more prone to a resulting leak. Although it has been determined that the frit may not fully enter into the molybdenum turns, conventional thinking was that it was undesirable to permit a gap between adjacent turns of the overwind.
A need exists therefore to reduce the extent of seal voids, and thereby leading to improving lamp life.